This invention relates generally to signal processing and, in particular, to signal correlation and cross-correlation using tapped optical fibers.
As fiber optic technology continues to grow, the use of various diffraction gratings within the fibers for signal processing will become increasingly common. Our co-pending U.S. patent application Ser. No. 09/765,215, incorporated herein by reference, describes a two-dimensional, fiber-delay radiator (FDR). The device is made with optical-fiber taps forming Bragg gratings orientated at 45 degrees to the fiber core, permitting light to emerge directly out of the side of the fiber, linearly polarized. The optical fiber is wound on a cylindrical-like form such that a number of loops of the fiber are available for making a number of taps on each loop. Taps are preferably generated along each loop of the fiber so that a portion of the light propagating in the fiber will exit sideways from fiber at the taps. A lens system is then used to capture the light from the taps and produce a Fourier Transform of the total distribution of light from all the taps. A video camera then captures this Fourier Transform light and the video signal is acquired for further computer processing. The result is that the power spectrum of the light signal is displayed on a monitor.
The preferred construction of the FDR would include taps with xe2x80x9cidealxe2x80x9d phase characteristics. It may be possible to directly fabricate such a device. However, a method to correct the phases in real-time and to introduce desired weighting may be preferable. To achieve this goal, a phase spatial light modulator may be used to correct and modify the tap phases. Another embodiment uses a coherent reference wave to generate a holographic optical element (or complex spatial light modulator) to correct the tap phases and amplitudes. According to this technique, a coherent reference source and a detector array are used to capture the radiation amplitude pattern of the FDR. Then, with digital processing of the captured pattern, the desired spectral signal properties are obtained.
This basic configuration finds application in a wide variety of commercial, industrial and military situations including, but not limited to the analysis of signals that have been modulated on light propagating in a fiber; the analysis of telecommunication channels in optical fiber network links; the analysis of optical sources by determining their spectral characteristics; and as part of a spectrometer to analyze gases and other materials. The spatial modulator version, in particular, could be used as a phased array for Ladar and Lidar applications; as a beam director for data storage and other scanning applications; and as a channelizer for use in separating out and monitoring individual signal channels.
This invention extends the principles disclosed in U.S. patent application Ser. No. 09/765,215, with emphasis on the use of a Tapped Optical-Fibers Processor (TOP) for correlating wide bandwidth and large time-bandwidth signals. In particular, it can be applied to processing radar and SAR (synthetic aperture radar) signals. The TOP can also be used for lidar and ladar systems with out the need for modulators or separate carrier light source. Broadly, using this approach, fine resolution can be obtained without fast front-end sampling and digital computational burdens can be significantly reduced.
A preferred method of signal processing according to the invention includes the steps of providing an optical carrier signal; modulating the optical carrier signal with an input signal to provide an optically modulated signal; radiating the optically modulated signal from a set of taps formed in an optical fiber; performing a spatial Fourier transformation on the radiated signal; detecting the Fourier transformed signal and converting the detected signal into an electrical signal; and performing a digital Fourier transformation as well as other processing functions on the electrical signal to output an autocorrelation of the input signal.
The radiated signal is a function of the distance between the taps, the velocity of the signal through the fiber, and an aperture weighting function. The resulting the autocorrelation of the input signal is weighted by the autocorrelation of the aperture weighting function.
Particularly in conjunction with radar signal processing, the input signal may be composed of the sum of at least two or more signals, in which case the output may include the autocorrelations of both inputs as well as the generation of cross-correlation of the two signals.
In terms of hardware, a signal processor according to the invention preferably includes a coherent laser source operating at a carrier frequency; a modulator to insert an input RF signal into the carrier; an optical fiber radiator composed of a fiber with taps that radiate the modulated optical signal; a lens to perform a spatial Fourier transformation on the radiated signal; and a detector array to output the transformed signal to a digital processor for additional signal processing. In any case, the two input signals may be electronically or optically combined.
In the case of ladar and lidar applications, the received light along with the source light could be used as input to the invention without the need for modulators or a separate carrier light source.